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PRACTICAL ORGANIC CHEMISTRY

It will be easily understood from these formula why the /3-com-
pound should yield benzonitrile with acetic anhydride whilst the a-
compound does not. The proximity of hydrogen and hyclroxyl in
the former case facilitates the formation and elimination of water.
In this way the configuration of most of the aldoximes may be
ascertained.

PREPARATION 90.

Benzoic Acid.The oxidation of the side-chains in aromatic
hydrocarbons is a matter of considerable interest, as illustrating
the difference of stability of the side-chain and nucleus, and also
the influence which the relative positions of the side-chains,
where more than one is present, exert in presence of oxidising
agents.

The oxidation of the side-chain of an aromatic hydrocarbon,
when more than one is present, takes place in successive steps.
Thus, mesitylene is converted into the following compounds on
oxidation ;

. C0H3(CH3)3CO.OII    Mesitylenic acid.

Mesitylene, C6H3(CU3)3 > CGH3CH3(CO.OJ I)a    Uvitic acid.

CH3(CO.OH):,

Trimesic acid.

The reagents usually employed are (i) chromic acid or
potassium bichromate and sulphuric acid, (2) dilute nitric acicl
and (3) potassium permanganate in alkaline or neutral solution.
The action of these upon the side-chain, when more than one
side-chain is present, depends upon their relative position.
Thus, for example, potassium bichromate and sulphuric acicl
either does not act, or completely destroys the compound when
the side-chains occupy the ortho-position (Fittig), whereas the
para- and meta-compounds yield the corresponding carboxylic
acids. ' This is true also of substituted hydrocarbons with one
side-chain ; thus with nitric acid m- and j^-nitrotoluene give
m- and ^-nitrobenzoic acid, whilst the ortho-compound is either
unattacked or destroyed. If, however, the substituent is a
halogen and the oxidising agent nitric acid, the m eta-compound
is least, and the para-compound most acted on. Dilute nitric
acid or alkaline permanganate are most serviceable for oxidising